Platelet adhesion to damaged blood vessel walls is facilitated by the binding of von Willebrand factor (VWF) to platelet membrane glycoprotein Ib (GPIb). This research proposal is a study of the biochemistry, cell physiology, and clinical significance of the GPIb receptor for VWF. The specific aims of this research program are: 1) Biochemistry: Characterize the binding site for VWF on GPIb. We have recently demonstrated that glycocalicin, a proteolytic fragment of GPIb, contains the VWF binding site. We now propose to characterize the binding site for VWF on glycocalicin by assessing digestion products of purified glycocalicin in assays developed to test glycocalicin/VWF binding. 2) Cell Physiology: Test the hypothesis that the GPIb receptor for VWF is processed from the interior of the platelet to the platelet surface and thence into the plasma. New methods have been developed which utilize a monoclonal antibody to quantify the GPIb receptor for VWF: a) inside the platelet (an enzyme-linked immunosorbent assay (ELISA) of the aqueous phase of Triton X-114 solubilized platelets), b) on the platelet surface (fluorescence-activated cell sorting (FACS)), and c) in the plasma (ELISA). Changes in these receptor pools will be determined after activation of washed platelets and storage of platelet concentrates. 3) Clinical significance: Investigate the GPIb defect in inherited platelet disorders. i) Wiskott-Aldrich syndrome (WAS): Preliminary studies with FACS have suggested tht patients with WAS have two subpopulations of platelets: one GPIb-positive and the other GPIb-negative. We now propose to characterize the GPIb defect in WAS patients and carriers. ii) Bernard-Soulier syndrome (BSS): BSS platelets are known to lack surface GPIb receptors for VWF. In order to clarify the underlying defect in BSS, the intraplatelet and plasma pools of this receptor will be analyzed. The results of these experiments are likely to provide a better understanding of the pathophysiology of platelet adhesion to damaged vessel walls. Clinically applicable knowledge which may be gained from this research program includes: changes in the adhesive properties of platelet concentrates during storage; metholology for diagnosis of carriers of WAS; and new insights into the pathogenesis of atherosclerosis, in light of the possible role of VWF in this disorder.